Luminescent solar concentrators (LSCs) collect and concentrate sunlight for use in solar power generation. LSCs are devices typically consisting of a planar waveguide coated or impregnated with a luminophore. Sunlight absorbed by the luminophore coated on or contained within the waveguide is re-emitted into the waveguide, where it is captured by total internal reflection, which causes it to travel to the edges to be concentrated for use by light conversion devices, such as photovoltaic cells (PVs). Unlike lens- and mirror-based concentrators, which require tracking systems to follow the sun's motion and can only concentrate direct, specular sunlight, LSCs are passive devices that work equally well with both diffuse and specular sunlight. They are therefore less costly to build, install, and maintain, more easily integrated into the built environment or portable solar energy systems, more damage tolerant, and can be used in climates where there is little direct sunlight. Furthermore, because LSCs can produce wavelength-to-bandgap matched photons by downshifting, there is reduced need for PV cooling, and multiple LSC waveguides each incorporating a different luminophore can be stacked to split the solar spectrum for tandem multi-cell conversion.
An exemplary LSC 10 is illustrated in FIG. 1, wherein the planar waveguide 12 comprises a plurality of luminophores 15. The planar waveguide 12 is edge-coupled to PV cells 14 sensitive to the emission wavelength of luminophores 15. As shown in the detail of FIG. 1, luminophores 15 absorb light 16 of a first wavelength, and emit light 18 of a second, red-shifted, wavelength. The emitted light 18 is used to generate electrical current in the PV cells 14.
In combination with bandgap-matched, high-efficiency PV cells, LSCs offer the potential for a reduction in the cost of solar electricity—by well over an order of magnitude. However, LSCs have thus far had little practical impact, primarily because the optical quantum efficiency (OQE) decreases rapidly with concentrator size (i.e., as an LSC increases in size, a smaller fraction of incident sunlight is concentrated at the edges). Several factors contribute to a decreasing OQE, two of which are usually dominant: (i) photon loss due to non-unity photoluminescence quantum yield (QY) of the luminophore and (ii) loss of photons from a top and bottom of the waveguide from emission at an angle inside the critical escape cone of the waveguide material, as defined by Snell's Law. For example, a typical organic luminophore-based LSC and a poly(methylmethacrylate) or glass waveguide can have a loss rate due to factor (i) that is near zero, but a loss rate due to factor (ii) that is about 25% per-emission. A decrease in OQE can also occur due to re-absorption and re-emission, as a captured photon traveling toward a waveguide edge may encounter other luminophores to be re-absorbed and re-emitted multiple times, and a fraction of photons can be lost with each successive re-absorption/re-emission event due to non-radiative relaxation processes. Repeating escape cone and QY losses thus compound with distance.
Previous efforts to address this problem have included the use of large-Stokes-shift luminophores that have a large energy difference between absorption and emission to reduce light re-absorption. Examples of such luminophores include certain organic dyes, quantum dots, lanthanide and transition metal-based molecules, and microcrystalline phosphors, and organic dyes whose emission is red-shifted by solid-state solvation. However, these luminophores tend to have low QY, narrow or weak spectral absorption bands that capture only a small portion of the solar spectrum, limited environmental lifetime, large scattering cross sections, or a combination of these shortcomings. Other methods for improving efficiency include the use of wavelength-selective mirrors and oriented luminophores for directing a larger portion of luminescence into waveguide modes and out of the escape cone. However, none of these approaches has proven successful in producing large area, high efficiency LSCs with long environmental lifetimes.
Accordingly, a large area, high efficiency LSC with a long environmental lifetime is needed. The present disclosure seeks to fulfill this need and provides further related advantages.